The invention relates to an apparatus for lithography or intaglio printing, and more particularly to an apparatus suited to intaglio printing effected without aid of dampening water or a doctor blade.
There are many forms of apparatus and methods designed to directly print papers or other sheet materials by means of a printing plate which is not dampened with water. This printing plate is covered with an oil-repellent laminate some portions of which are eliminated to form a lipophilic image area, other portions of the laminate remaining to form an oil-repellent non-image area. The oil-repellent laminate is composed of a silicone rubber carried on an elastic sheet and having photosensitive chemical groups in its molecular structure. The above-mentioned partial elimination of the oil-repellent laminate is effected photographically by means of exposure to a light image and development by a developer solution. Such an intaglio printing plate is then mounted on a plate cylinder in order to print thick sheet materials such as metal or plastic sheets. The printed thickness of of an ink produced in this manner is much greater than in normal printing methods so that the printed thick materials thus produced can be used as printed electric circuits or name plates.
It has, however, been observed that it was difficult to transfer a sufficient amount of ink from the printing plate to the metal or plastic sheets. This problem in such a printing technology (hereinafter called "thick printing") seems to originate from a poor ink-absorbability of the metal or plastic sheets and also from a high viscosity of the ink used therein, the high viscosity being necessary to avoid an undesirable lateral expansion of the ink on said sheets.
To solve the above problem, it has been proposed to reduce a rotational speed of the printing plate to a range of 200 to 300 mm/sec. This speed in the thick printing is equal to about one tenth of that in the ordinary printing. A clear and vivid image will be produced on the metal or plastic sheets owing to sufficient deposit of ink thereon, if the following difficulty were not encountered.
The difficulty with a reduced speed of the printing plate is that it results in an excessive amount of ink transferred from the inking rollers to said printing plate. This superfluous transfer of ink is most disadvantageous in the case where neither dampening water nor a doctor blade is used because the comparatively lower oil-repellency of said silicone laminate brings about a contamination of the non-image area thereby making the printed sheets dirty.
Furthermore, known printing systems have still another disadvantage in terms of economy. In cases where it is necessary to alter the dimensional sizes of the printing plates, the known systems must use plural plate cylinders of various sizes or comprise a single cylinder of an extremely large diameter capable of attaching thereon any large printing plate and any small printing plate. When a small plate is to be attached on the large diameter cylinder having a plate holding device located thereon at a fixed position, the small plate must be enlarged to have a great portion of blank area so that manufacturing cost of such small printing plates, especially intaglio plates, becomes very expensive and uneconomical.